Film, television and similar imaging systems portray moving images by presenting a viewer with a sequence of still images that correspond to temporal samples of a scene. The rate of presentation of new images and the duration of presentation of each image are crucial factors in determining the subjective quality of the viewing experience. Historically the rate of presentation of new images was identical to the rate of image acquisition in a camera; and, the duration of presentation was limited by available display system technology. In modern image processing systems it is possible for these display characteristics to be determined independently of the parameters of the image acquisition, storage and distribution mechanisms.
Where moving images are acquired by photography the temporal sampling frequency (i.e. the frame rate) is limited by the mechanical processes of film transport. This has lead to the widespread adoption of a temporal sampling frequency of 24 Hz (frames per second). In television there is no mechanical limitation, but it is still necessary to allow sufficient “exposure time” for each image to be acquired by the electronic sensor; temporal sampling frequencies (field rates) of 50 Hz and 60 Hz are common.
In mechanical film projection the duration of presentation of each image is less than the temporal sampling period (because of the time for film transport); however, because of the relatively-low temporal sampling rate, each image is repeated (flashed) one or more times to make the resulting flicker imperceptible. In television, because of the higher temporal sampling rate, each image is usually only displayed once. There are thus significant subjective differences between the film and television viewing experiences.
Further complications arise when film-originated material is displayed via television. When (as in Europe) the television field rate is approximately twice the film frame rate, normal practice is to adjust the film frame rate (with possible change to the duration of the material) to equal the televisions frame rate, so that each film frame is displayed twice (as two consecutive television fields). In countries where the television field rate is approximately 60 Hz (such as the USA) film frames are shown in a ‘3:2’ sequence in which alternate film frames are shown three times and twice, respectively. This is a compromise which gives an inferior viewing experience.
These methods of presenting film via the television medium thus give a viewing experience that may be substantially different from that intended by the cinematographer and represent, in varying degree, a distortion of the intended viewing experience.
It is common to combine film-originated and television-originated material in the same television presentation. Often, the difference in viewing experience due to the different method of image acquisition represents an undesirable distraction for the viewer. It often preferable for the entire presentation to have either, the ‘film-look’ or the ‘television look’ according to the intention of the director. In other cases it may be desirable to artificially increase the subjective difference between different parts of the same presentation for artistic reasons.
It is thus advantageous to be able to control the subjective viewing experience of televised material in order to achieve a desired type of motion rendition. Known systems include temporal interpolation (sometimes including motion-compensation) that up-converts from the temporal sampling frequency of the film frames to the temporal sampling frequency of the television system. Usually the up-conversion is to the field rate of an interlaced television system; such conversion may be performed optically, for example in a ‘polygon’ telecine in which light from more than one film frame is combined in a rotating optical prism.
However, these prior-art systems do not accurately replicate the motion rendition of film projection in a television or other electronic moving image rendition system.